1. Field of the Invention
The present invention relates to a synchronous rectifier circuit adopting a synchronous rectifying system, which is one type of switching power circuits in which input power is outputted after interrupting the input power in proportion to a predetermined output and after smoothing, and particularly relates to a synchronous rectifier circuit whose efficiency is not lowered even in light-load operation.
2. Description of the Background Art
Switching power circuits have been widely used conventionally as small power circuits having high efficiency. The switching power circuits output an input power after interruption and smoothing, and adjust the proportion of the input power when interrupting in accordance with an output voltage or output current, thus supplying a voltage or current of a constant value to a load, regardless of a change in load.
In recent years, various methods have been proposed to further improve the efficiency of the switching power circuit, and for example, Japanese Unexamined Patent publication No. 261950/1997 (Tokukaihei 9-261950) discloses a switching power circuit (synchronous rectifier circuit) adopting a synchronous rectifying system.
As shown in FIG. 20, in a conventional synchronous rectifier circuit 101, input voltage Vin inputted to input terminal IN is applied to output terminal OUT via an inducing element 112 and a first switch 111 while the first switch 111 is being conducted. In order to maintain output voltage Vout constant, the output terminal OUT is grounded via a smoothing capacitor 113.
Under this condition, the inducing element 112 stores an energy, and current I.sub.L flowing through the inducing element 112 in a direction towards the output terminal OUT is increased, as shown in FIG. 21, with the slope of (Vin-Vout)/L (period between ta and tb in FIG. 21).
Meanwhile, a commutating diode 114 and a second switch 115 are provided parallel to a series circuit composed of the inducing element 112 and the smoothing capacitor 113. When the first switch 111 is cut-off (at time tb), the current I.sub.L flowing through the inducing element 112 is maintained by the commutating diode 114 and the second switch 115 being conducted. Under this condition, the energy stored in the inducing element 112 is released, and the current I.sub.L is reduced with the slope -Vout/L (period between tb and te) . At time te, the first switch 111 is conducted again, and the inducing element 112 starts storing energy.
The first and second switches 111 and 115 are controlled by a control circuit 121. The control circuit 121 controls the ratio of conduction period to cut-off period of the first switch 111 by monitoring the output voltage Vout so that the output voltage Vout takes a certain value. Here, when the first switch 111 and the second switch 115 are conducted at the same time, the input terminal IN is grounded via the first and second switches 111 and 115, and as a result an extremely large feedthrough current flows. Thus, the control circuit 121 provides a predetermined dead time Tdet between the switching timing of the first switch 111 and the switching timing of the second switch 115 so that the first switch 111 and the second switch 115 are not conducted simultaneously.
In this structure, while the second switch 115 is being conducted, the current I.sub.L flowing through the inducing element 112 flows mainly through the second switch 115, and essentially no current flows through the commutating diode 114. Therefore, even in heavy-load operation in which output load current Iout is large, a forward voltage loss due to the commutating diode 114 is not generated, thus realizing synchronous rectifier circuit 101 having superb efficiency.
However, the synchronous rectifier circuit 101 having the described structure has a problem that the efficiency is often lowered in light-load operation. Specifically, in light-load operation, the load current Iout is notably small, and the control circuit 121 sets a long cut-off period of the first switch 111. As a result, there is a case where the first switch 111 is not conducted even after time tx at which all the energy stored in the inducing element 112 in the conduction period of the first switch 111 has been released. In such a case, contrary to the normal operation, the current flows from the output terminal OUT to GND via the inducing element 112 and the second switch 115. As a result, the conversion efficiency of the synchronous rectifier circuit 101 is reduced to 50 percent or less.
Note that, when the second switch 115 has a polarity, the reverse current can be prevented. However, when a MOSFET is adopted as the second switch 115, the current also flows in a direction opposite to the polarity by the body diode formed in the MOSFET. Here, for example, when a commutating element such as diode is connected in series to the MOSFET to eliminate the reverse current, the efficiency in heavy-load operation is reduced by the forward voltage of the diode.